7.2 Array Architectures

Quick Answer: Handbook v1.3 Learning Outcome 3.2.1 names five array architectures: (a) a single-string array, series/parallel wired, into a single inverter; (b) multiple-string with multiple inverters; (c) multiple-string into a single inverter with multiple MPPT; (d) multiple-string into a single inverter with a combiner box; (e) a central inverter with sub-arrays. Choose among them using shade, orientation, DC cable length, combiner isolation, inverter count and monitoring granularity. This OpenExamPrep section is independent study material for that 2922-34 list; it is not a City & Guilds publication.

Architecture is not the same as inverter type

Chapter 6 named inverter types (domestic string, commercial string, central, micro, hybrid). This section names how strings are grouped. The same garage string inverter can be used as architecture (a), (c) or (d) depending on whether you have one matching string, two MPPT inputs on unlike roofs, or several parallel strings landed through a combiner. A central inverter in the type list is the machine; architecture (e) is the sub-array drawing around it. Keep the two vocabularies separate on the paper.

Unit 301 puts nine questions (30 percent) on Learning Outcome 3. Architecture items are cheap if you can point to a roof and say which of (a)–(e) you have drawn, and expensive if you call every job a single string because there is only one inverter in the garage.

(a) Single-string, series/parallel wired, single inverter

One conversion machine. The modules are wired in series, or in a series-parallel combination that still presents as one array input — typically one tracker. A simple unshaded south pitch of twelve matching modules in series is the textbook (a). A small parallel pair of identical short strings landed on one MPPT is still (a) in handbook language: series/parallel wired, single inverter, not independent trackers.

Use (a) when every module sees similar irradiance, the same tilt and azimuth, and the same module count per series path. Shade on one module is then a string-level problem that relies on bypass diodes (section 7.1). Different orientations do not belong on architecture (a).

(b) Multiple-string, multiple inverters

Each string, or each group of strings, has its own inverter. That may be two garage string inverters, a row of commercial string machines, or a field of microinverters (conversion per module or small group — still multiple conversion machines). You gain independent MPPTs, independent monitoring, and a partial failure: one inverter dies and the other strings still produce AC.

You pay with inverter count: more AC isolators, more labels, more G98/G99 generating devices to add up, more firmware, more roof or loft electronics if they are micros. Two 3 kW inverters on an east face and a west face are architecture (b). They are a valid 2922 answer when the customer already owns two machines, or when one chassis cannot provide two honest trackers. They are not mandatory just because the bungalow is L-shaped.

(c) Multiple-string, single inverter, multiple MPPT

One conversion cabinet, two or more independent maximum power point trackers. East string on MPPT 1, west string on MPPT 2. Same plant-room unit, two I–V curves, two voltage windows. This is the usual cost-effective 2922 answer for a split roof that is otherwise unshaded on each face.

Monitoring is typically per tracker, not per module. Shade on the east string still hurts that string; the west tracker is unaffected. DC cable length is still a drop from each face to the same inverter location. Isolation is usually at the inverter DC inputs plus any extra string isolators the design requires. Inverter count stays at one, which keeps the AC connection simple.

Do not parallel the two unlike faces onto one tracker and call it (c). Multiple MPPT means independent trackers, not two labels on a paralleled bus.

(d) Multiple-string, single inverter, combiner box

Several strings of matching orientation and module count are brought to a combiner. The combiner parallels them onto one DC output that feeds one inverter input (often one tracker). The combiner is where you can put string fuses (section 7.1 / later OCPD chapter), string isolation, surge protection, and a single pair of thick DC cables onward.

DC cable length is the trade: short string cables on the roof, then a long combined DC run to a distant inverter — or a combiner next to the inverter with long string cables. Isolation at the combiner can kill several strings in one enclosure, which is convenient and also a common DC bus you must treat as live in daylight. Monitoring is often array-level unless the combiner or inverter reports per string. Shade or a different orientation on one combined string still fights the others on that paralleled bus.

Architecture (d) is the wrong default for east-plus-west unless each orientation has its own combiner and its own MPPT. A single combiner that ties unlike faces together is just architecture (a) with extra boxes.

(e) Central inverter with sub-arrays

Large sub-arrays — each a paralleled group, often after combiners — land on one high-power central inverter. MPPT, conversion and much of the protection sit at that central plant. Monitoring is coarse unless extra string-level hardware is added. DC cable length to the central machine can be long. Inverter count is one, but it is a commercial-scale one.

On the 3.2.1 list you must name it. On a house survey it is usually the wrong picture. Do not relabel a 4 kW garage string inverter as central because it is the only box on site. Central means sub-arrays into one big inverter, not there happens to be one cabinet.

Trade-offs the paper actually tests

Shade wants independent trackers (b) or (c), or module-level conversion inside (b). Different orientations are the same demand. DC cable length wants conversion near the modules, or honest voltage-drop design on a long DC pair. Combiner isolation is the (d)/(e) convenience and the (d)/(e) common-bus hazard. Inverter count is cost, AC connections and G98/G99 addition. Monitoring granularity is how small a fault you can see without a roof walk.

Scenario: L-shaped bungalow, east and west roofs

An L-shaped bungalow has a clear east pitch and a clear west pitch, similar kWp on each, no chimney bars. The customer wants one tidy plant cupboard.

Do not series-wire east and west as architecture (a). String current would follow the weaker face all day. Do not dump both faces into one combiner onto one tracker as architecture (d). That combiner is paralleling two different I–V curves.

The usual 2922 drawing is architecture (c): one inverter, two MPPTs, east on tracker 1, west on tracker 2. One AC connection. Independent orientation tracking. Remaining shade on one face still sits at string level on that tracker.

Architecture (b) — two inverters, or micros — is the upgrade when a face is itself chopped by shade, or when module-level monitoring is specified. Architecture (e) is not a bungalow answer. The exam trap is treating split orientation as a reason to buy a central inverter, or as a reason to parallel unlike strings because there is only one garage wall.

Scenario: farm barn with a long DC run

A portal-frame barn has a large unshaded south roof and a plant room 60 m away at the house. Several matching strings will fit.

If you combiner-parallel those strings at the ridge and run one fat DC pair to a house inverter, you have drawn architecture (d) (or (e) if the machine is truly a central inverter with sub-arrays). You have short roof string cables, a long live DC run, combiner isolation at the barn, one inverter, and coarse monitoring. Voltage drop, DC isolation at both ends, and the reverse-current/OCPD story on those parallel strings all land on the design. EMI wants the + and − cables together; a 60 m loop is a problem, not a feature.

If you put a string inverter (or two) at the barn and run AC to the house, you have shortened the live-in-daylight DC. That may be (a) or (c) at the barn, or (b) if you use more than one machine. You have traded DC voltage drop for an AC cable, an outdoor IP rating, and service access in a dusty barn.

The competent 2922 choice is the trade-off table, not a slogan that long DC is always wrong or always right. Name the architecture, then say what you did to shade, cable length, isolation, inverter count and monitoring. A barn is not automatically architecture (e) just because it is agricultural.